Polymer hydrogenation, and the subsequent separation of the hydrogenation catalyst from the polymer, are both well known unit operations, as referred to (for example) in U.S. Pat. Nos. 4,396,761; 4,510,293 and 4,595,749.
More specifically, certain rhodium-containing catalysts are known to be particularly suitable for the "selective" hydrogenation of nitrile rubber (i.e. the selective hydrogenation of the carbon-carbon double bonds without hydrogenation of the carbon-nitrogen triple bonds).
U.K. patent No. 1,558,491 teaches the use of chlororhodium (tris triphenylphosphine), RhCl(PPh.sub.3).sub.3, in a process to hydrogenate unsaturated nitrile rubber. U.S. Pat. No. 4,464,515 teaches the use of hydrido rhodium tetrakis (triphenylphosphine) catalyst, HRh(PPh.sub.3).sub.4, in a process to selectively hydrogenate unsaturated nitrile rubber. In both of these known processes, the unsaturated nitrile rubber is first dissolved in a suitable solvent to provide a viscous rubber solution. The catalyst is then dissolved in the rubber solution. These hydrogenation processes are said to be homogeneous because the substrate and catalyst are contained in the same phase.
The selectively hydrogenated nitrile rubber which is produced by either of the above homogeneous hydrogenation processes is less susceptible to heat-induced degradation in comparison to unsaturated nitrile rubber. One advantage of the above homogeneous processes is that they are efficient with respect to the minimal amount of catalyst which is required to catalyze hydrogenation of the carbon-carbon double bonds. Conversely, a disadvantage of a homogeneous catalytic process is that it is generally quite difficult to remove the catalyst from the system when the reaction is completed (in comparison to a heterogeneous process, where the catalyst is not dissolved and hence may be readily removed by filtration or centrifugation).
As rhodium is an active catalytic metal, it is desirable to remove it from the hydrogenated rubber to improve the quality of the final product. Furthermore, the high price of rhodium provides an economic incentive for the recovery of rhodium. There is very little prior art directed towards the recovery of rhodium from hydrogenated rubber. The exception is co-pending U.S. application Ser. No. 226,883 (filed August 1988), now U.S. Pat. No. 4,944,926, which teaches a method to treat rhodium-containing solutions derived from the hydrogenation of nitrile rubber. However, the method of the aforesaid application requires the use of a finely divided organic additive, which additive has the potential to contaminate the hydrogenated nitrile rubber product if it is not completely removed.
It is also known to recover rhodium complexes from non-viscous chemical process streams using ion exchange resins.
Chemical Abstracts ("CA") 75: 10878e (1971) describes the separation of rhodium-containing catalysts from oxo reaction streams using ion exchange resin.
CA 85: 588k (1976) teaches the use of a thiol-functionalized resin to recover Group VII metals from spent organic solutions which contain catalysts.
CA 87: 26590p (1977) describes a two stage process in which (i) an aqueous, noble-metal-containing solution is prepared by extracting metal from a catalyst carrier and (ii) the noble metal is adsorbed by ion exchange resin.
CA 95: 10502r (1981) relates to the recovery of Pt and Rh by extracting the metals from spent catalysts using HCl and HNO.sub.3, followed by the subsequent use of an ion exchange column to separate the metals.
Heretobefore, there has not been disclosed the removal of rhodium from hydrogenated nitrile rubber solutions using a selected ion exchange resin. The present method does not require the use of a finely divided additive and provides excellent recovery efficiencies.